Polyimides have excellent acceptance in demanding applications such as aerospace, traction motors and flexible printed circuitry because of their unique combination of physical, electrical and chemical properties. Polyimide articles, such as films, are usually manufactured in a two-stage process. The first stage is a polycondensation reaction between a dianhydride and diamine in a suitable solvent. The resulting polyamic acid solution is then fabricated into a shaped article, e.g., a film, usually by solvent casting. In the second stage, the polyamic acid film is either converted chemically or thermally to the polyimide by elimination of water.
Polyimides derived from a 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane are useful, inter alia, for the production of insulating varnishes and electronic coatings on semiconductor wafers, such as polysilicon wafers as disclosed in application Ser. No. 515,223 filed Apr. 27, 1990. As noted in that application, one way of forming such coating on the wafers is to apply a solution of a polyamic acid polymer derived from such 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane to the wafer and thereafter bake the coated wafer to cure (imidize) the resin so that the corresponding polyimide is formed.
During this curing, two main proceses take place: removal of solvent and a chemical conversion of the polyamic acid to polyimide. Both these processes result in loss of film volume and can produce stresses in the film if the losses occur after the film has become (semi-)rigid. In addition, stresses in the film can arise when these is a mismatch between the coefficient of expansion of film and substrate as the film cools from its curing temperature to room temperature. Upon exposure to certain organic solvents, these stresses visibly appear in the coatings as breaks in the film. Such breaks are typically referred to as cracks or crazes. According to the aforesaid Application, this problem is addressed by adding another diamine, a diamine having aliphatic substituents.
U.S. Pat. No. 4,851,505 discloses polyimidies which contain such diamines having aliphatic substituents and in which the carboxylic moiety is tetracarboxyl biphenyl. The polyimides yield optically transparent films which can be cast from solution.